In certain mechanical industries, and in particular in the automobile industry, new parts and new casting methods are being developed which necessitate the provision of non-destructive inspection means in order to evaluate the parts produced to new designs or obtained by new methods. In particular, it is necessary to carry out qualitative and quantitative inspections as to the material of the part in order to determined whether any defects are present as well as dimensional and geometric inspections in zones of the parts which may be inaccessible by conventional means, for example in cavities or channels inside the part.
In particular, in the case of castings of aluminium alloy or of cast iron which are used for the construction of automobiles, such as cylinder heads, crankcases, crankshafts or steering column housings, it may be necessary to carry out non-destructive inspections or dimensional inspections on batches of parts which have been obtained using new designs or by new casting methods.
In so far as castings are concerned, the defects present in the parts can be detected by X-ray radioscopy, by gamma radiography or by ultrasonic inspection. These inspection means give indications as to the presence or absence of defects in the part, but the judgement of the conformity of the part and the soundness of its material is above all qualitative.
The geometric and dimensional inspections must be carried out in a destructive manner when it is a question of checking the shapes and dimensions of internal portions of the parts. In this case parts taken from manufacture are cut in such a way as to reveal the elements to be inspected and conventional dimensional measurement tools such as callipers or micrometers are used.
In order to obtain virtual images of sections of a mechanical part, it is known to use industrial tomography devices. Such devices including a radioscopy installation, for example using X-rays, means for relative displacement of the mechanical part in rotation with respect to the radioscopy installation and means for utilising the images in order to obtain tomographic sections are very expensive and are difficult to integrate into a castings production environment.
All the industrial tomographic methods are based on the principle of acquisition and processing of images of the part which are obtained by radioscopy.
The radioscopy installation essentially comprises a source of radiation, for example X-rays, with sufficient power to pass through the mechanical part and a luminescent screen such as an image intensifier. The part is interposed between the radiation source and the screen in such a way that the radiation can pass through it, the intensity of the radiation being attenuated and modulated as it passes through the part as a function of the opacity or the density of the materials through which it passes. The radiation having passed through the part produces an image on the screen which is representative of the material through which the radiation has passed. Generally an analogue or digital camera makes it possible to acquire the radioscopic images formed on the screen.
In order to obtain virtual tomographic sections of the part along sectional planes which are parallel to one another, acquisition of radioscopic images of the part is effected according to a very large number of successive orientations which are obtained by making the part turn about an axis perpendicular to the tomographic sectional planes.
The radioscopic images in digitised form are processed in a calculator using algorithms which are all based on the same principle, consisting of carrying out a Radon transform with filtered back-projection of the images of the part, on the basis of which a reconstruction of the images is carried out in order to obtain the tomographic section.
At least one image is produced for each of the relative elementary rotations between the part and the radioscopy installation, followed by processing of the images in order to obtain a synogram, this intermediate image then being reconstructed in order to obtain the virtual image which constitutes the tomographic section.
The synogram is obtained by juxtaposing the lines of the images corresponding to the sectional plane in which the tomographic view is produced.
Several tomographic views may be produced corresponding to successive sections of a portion of the part in the direction perpendicular to the sectional plane in such a way as to reconstitute the shape of the part in three dimensions.
The method of scanned radioscopy which enables sections to be obtained through a body or object is used both in the industrial field and in the medical field.
In the case of the scanner for medical use, the whole of the radioscopy device is made to turn around the patient in order to produce the successive images which are utilised in order to reconstruct the tomographic sections.
In the case of inspection of industrial parts, the part is generally fixed on a manipulator for adjustment of its position and for its displacement in rotation.
In all cases, it is necessary to use expensive and cumbersome installations which are generally fixed permanently in a building reserved for inspection.
In the case where it is desired to carry out inspections of castings on samples coming from manufacture, such devices which are cumbersome, expensive and permanently installed are not suitable.
On the other hand, conventional radioscopy installations are generally available in workshops for production of parts in order to check for defects in samples of parts from manufacture.
Such installations have limited performance and in particular do not permit inspection of the soundness of the material from which the part is made in sectional planes passing through the part.
Equally, the conventional methods of radioscopy do not permit non-destructive dimensional or geometric inspections to be carried out inside the parts.
More generally, on the production lines for industrial products which may be for example parts or any other product, practical means are not usually available which enable tomographic sections of the products to be carried out.